1. Field of the Invention
The present invention, in the field of microscopic processing technology for semiconductor fabrication and the like, relates to a pattern formation method, which forms a predetermined pattern in a resist film upon a wafer through exposure using phase-shift masks. Hereinafter, exposure using phase-shift masks is abbreviated as xe2x80x9cphase-shift exposurexe2x80x9d.
2. Description of the Related Art
In recent years, miniaturization in pattern dimensions have become increasingly necessary as achievements in high-speed and high levels of integration in semiconductor devices are made. As a result, design rules have become reduced to approximately half the exposure wavelength.
For example, FIG. 8 shows optical contrast in the case where, for example, a 100 nm isolated line is exposed onto a photoresist layer through KrF excimer exposure (wavelength 248 nm) or ArF excimer exposure (wavelength 193 nm). The 100 nm isolated line is a line that has a plurality of linear pattern elements of width 100 nm arranged in parallel at equal intervals. Optical contrast is defined as (light intensity at pattern centerxe2x88x92light intensity at pattern edge)÷(light intensity at pattern edge), whereby it is believed that a value of approximately 0.5 or more is necessary for resolving a pattern into a preferred shape. On the other hand, in formation of patterns with width of approximately 100 nm, pattern width is less than half the exposure wavelength. As can be seen from FIG. 8, in formation of patterns with microscopic widths to this degree, resolving into the preferred shape using a normal mask subjected to exposure techniques is extremely difficult since the optical contrast is smaller than 0.5. Thus, various xe2x80x9csuper resolution techniquesxe2x80x9d are under review. Among them, the xe2x80x9cLevinson phase-shift maskxe2x80x9d (see Japanese Patent Application Laid-open Sho 62-50811) is considered as the most promising technique in formation of patterns less than half the exposure wavelength since the optical contrast and resolution improving effects are significant.
FIG. 9A to FIG. 9C are top views illustrating a pattern formation method through conventional phase-shift exposure, wherein FIG. 9A is a phase-shift mask used in a first exposure, FIG. 9B is a normal mask used in a second exposure, and FIG. 9C is a circuit pattern formed through these exposures. Width Ws of the line patterns shown in FIG. 9C is 100 nm. Description thereof is based on these drawings hereinafter.
A phase-shift mask 40 shown in FIG. 9A is used in the first exposure. The phase-shift mask 40 is a positive type Levinson phase-shift mask, and has an L-shaped light shielding portion 421b and linear light shielding portions 421a. The phase-shift mask 40 has a phase-shifter 422b, which is formed adjacently to the L-shaped light shielding portion 421b, and phase-shifters 422a, which are alternately arranged in the spaces between the linear light shielding portions 421a. The region other than the L-shaped light shielding portion 421b, linear light shielding portions 421a, phase-shifter 422b and phase-shifters 422a of the phase-shift mask 40 is made to be a transmissive portion 423. Since the phases of the exposure light that passes through the transmissive portion 423 do not change, xe2x80x9c0xe2x80x9d is given to the transmissive portion 423 in FIG. 9A; and since the exposure light that passes through the phase-shifters 422a and 422b changes in phase only by xcfx80 (180), xe2x80x9cxcfx80xe2x80x9d is given. The phases of the exposing light that passes through the transmissive portion 423 adjacent to the shielding portions 421a and the exposing light that passes through the phase-shifters 422a differ precisely 180. During the first exposure, the photoelectric fields at the borders of the transmissive portion 423 with the phase-shifters 422a and 422b are completely neutralized, forming an image with an extremely sharp dark area. The dark area formed at the 0-xcfx80 border is called a phase edge. The second exposure subsequently performed is exposure for preventing unnecessary dark areas including the phase edge from being resolved as resist patterns. In other words, using the normal mask 42 shown in FIG. 9B, all of the line pattern regions and L-shaped pattern regions formed through the first phase-shift exposure are shielded from the light so as to expose the remaining regions. In particular, the unnecessary dark area formed at the 0-xcfx80 border is exposed to be eliminated. A circuit pattern 44 is then achieved upon a wafer 441 by executing a development procedure.
However, there are problems such as the following in a conventional pattern formation method through phase-shift exposure.
FIG. 10 shows what happens to dimensions (width Ws) of a pattern to be actually formed in accordance with the distance between adjacent patterns (inter-pattern distance) W in the case where a line pattern where width Ws is 100 nm is exposed by phase-shift exposure. As shown in FIG. 10, pattern dimensional accuracy drastically decreases due to affects of the optical proximity effect with the inter-pattern distance W equal to or less than 400 nm.
Based on a fixed value for the inter-pattern distance W, a pattern with inter-pattern distance W larger than the fixed value is defined as an isolated pattern 461, and a pattern with inter-pattern distance W smaller than the fixed value is defined as a dense pattern 462. Conventionally, as shown in FIG. 9C, the isolated pattern 461 with the large inter-pattern distance W and the dense pattern 462 with the small inter-pattern distance W have been simultaneously exposed with the same phase-shift mask 40. At this time, should the isolated pattern 461 and the dense pattern 462 be exposed as line patterns with same dimensions, there have been problems where dimensional differences of the pattern actually formed significantly increase. For example, as shown in FIG. 10, if a pattern where width Ws is 100 nm is exposed, a 40 nm dimensional difference between the pattern actually formed with the isolated pattern and dense pattern arises. Correcting (optical proximity correction) such large dimensional differences on the reticle side is extremely difficult.
Furthermore, with conventional phase-shift exposure, as shown in FIG. 11, there have been problems where pattern dimensions drastically thicken as defocus, that is wafer surface asperities, increases. Thus, pattern dimensions vary in response to the irregular structure of the wafer surface.
Moreover, as in FIG. 12, since asymmetry in +, xe2x88x92 defocusing has occurred when spherical aberrations persist, there have been problems of dimensional accuracy significantly deteriorating. Asymmetry in +, xe2x88x92 defocusing indicates that even if the absolute value of the plus defocus for a given point and the absolute value of the minus defocus for another point is the same, the pattern dimensions at these points do not match. By constructively utilizing phase information under highly coherent conditions for image formation, phase-shift exposure becomes extremely sensitive to optical parameters on that principle. Consequently, the exposure is extremely affected particularly by the effects of lens aberrations that emerge as phase errors during image formation.
Accordingly, the aim of the present invention is to provide a pattern formation method through phase-shift exposure, which improves dimensional accuracy by eliminating effects of defocus and spherical aberrations in addition to affects of the optical proximity effect.
The present invention is a pattern formation method, which forms a predetermined pattern in a resist film upon a wafer through exposure using phase-shift masks, characterized by using differing phase-shift masks in response to an inter-pattern distance, which is the distance between adjacent patterns, and exposing under respective adequate conditions for respective phase-shift mask exposures. For example, reducing the amount of exposure as the inter-pattern distance becomes shorter, or exposing using focus offset that differs according to the inter-pattern distance. Furthermore, a first phase-shift mask with inter-pattern distances of at least a fixed value and a second phase-shift mask with inter-pattern distances less than the fixed value may also be used. The optical proximity effect depends on inter-pattern distance as well as exposure conditions. Accordingly, using phase-shift masks that differ according to inter-pattern distance, providing optimum exposure conditions for respective inter-pattern distances allows for eliminating affects of the optical proximity effect.
Regarding the differing phase-shift masks, one phase-shift mask may have a mask pattern that eliminates the phase edges generated in another phase-shift mask. In this case, since the phase-shift mask also has functions of the normal mask, the normal mask is eliminated.
Furthermore, at least one of either the phase-shift mask or the wafer may be exposed while being shifted a fixed distance along the optical axis. In the case where the first phase-shift mask is used, at least one of either this first phase-shift mask or the wafer may be exposed while being shifted a fixed distance along the optical axis. Moreover, the fixed distance may be determined in response to the amount of spherical aberrations of the exposure projection lenses. In this case, the pattern dimensional errors caused by defocus and/or spherical aberrations are averaged.
In other words, the present invention provides a pattern formation method characterized by dividing a Levinson phase-shift mask pattern into an isolated pattern portion and a dense pattern portion, and performing multifocal point exposure for the isolated pattern portion, thereby reducing aerial image variances and aberration effects due to defocus as well as significantly reducing the optical proximity effect, consequently allowing for marked improvement in dimensional accuracy.